Depends on what you have in the tank? If you have a wall of LR, you want to be sure to get flow around, behind, and infront of the LR. There are different opinions on number of heads and direction, but I like to get a random flow of water by directing two different heads at one another from opposite sides of the tank. I also have a third head running accross the sand bed to try and eliminate any dead spots in the tank. I hope this helps